1. Field of the Invention
The present invention relates to a particulate detection system (hereinafter also referred to a “system”) for detecting the quantity of particulates contained in exhaust gas which flows through an exhaust pipe.
2. Description of the Related Art
The exhaust gas of an internal combustion engine (for example, a diesel engine or a gasoline engine) may contain particulates such as soot.
Exhaust gas containing such particulates is purified by collecting the particulates through use of a filter. When necessary, the filter is heated to a high temperature so as to burn particulates accumulated on the filter, to thereby remove the particulates. Therefore, when a failure such as breakage of the filter occurs, unpurified exhaust gas is discharged directly to the downstream of the filter.
Therefore, there has been a demand for a particulate detection system which can detect the quantity of particulates contained in exhaust gas in order to directly measure the quantity of particulates contained in (unpurified) exhaust gas or detect a failure of the filter.
For example, Patent Document 1 discloses a particular measurement method and apparatus. Namely, Patent Document 1 discloses a method of mixing an ionized gas which contains positive ions with exhaust gas which is introduced from an exhaust pipe into a channel and which contains particulates to thereby charge the particulates, and then releasing the charged particulates to the exhaust pipe. The method detects a current (signal current) which flows in accordance with the quantity of the released, charged particulates, to thereby detect the concentration of the particulates.
As described above, in the particulate detection system, a detection section is attached to an exhaust pipe, and exhaust gas is introduced into the detection section so as to detect particulates contained in the exhaust gas within the exhaust pipe. Therefore, a portion of the detection section is placed in a state in which that portion communicates with the inner space of the exhaust pipe.
[Patent Document 1] WO2009/109688
3. Problems to be Solved by the Invention
Since an internal combustion engine or an exhaust pipe is cooled after a previous operation of the internal combustion engine, depending on the outside air temperature, condensed water may accumulate within the exhaust pipe or the housing of a turbo charger. Therefore, for a short time after startup of the internal combustion engine, exhaust gas may contain water droplets. Also, condensed water may be present inside or around the detection section itself before startup of the internal combustion engine. That is, the detection section may be placed in a state in which water droplets adhere thereto before startup of the internal combustion engine or thereafter. Notably, the water droplets adhering to the detection section evaporate when, upon elapse of time from startup of the internal combustion engine, the temperature of the internal combustion engine increases, or the temperatures of the exhaust pipe and the detection section increase due to heating by exhaust gas.
However, in the case where water droplets remain on the detection section, depending on the position where the water droplets adhere to the detection section, the water droplets may lower the insulation resistance between the constituent members of the detection section. If a drive processing circuit starts drive of the detection section and applies a voltage thereto in a state in which the insulation resistance between the constituent members has been lowered, an undesirable current flows. Thus, the load acting on a power supply current within the drive processing circuit may become excessive. Further, operations such as discharge at the detection section become unstable, whereby proper detection may become impossible. Also, since water droplets adhere to the surface of an insulating member, which provides electrical insulation, a current flows between members which are to be insulated from each other by the insulating member, whereby migration occurs. In such a case, a current path is formed on the surface of the insulating member, and the insulation resistance is permanently decreased. Thus, a problem or failure such as degradation of the function of the detection section may occur.